1. Field of the Invention
This invention relates to an electrically insulating bearing.
2. Description of the Prior Art
A conventional bearing having a rolling element interposed between an outer ring and an inner ring has been known in the art. In rotary electrical equipment, shaft voltage is usually generated between both ends of a rotary shaft, between the rotary shaft and each bearing, or in sections of the rotary shaft engaging with each bearing. This shaft voltage is generated due to various reasons, such as magnetic unbalance, static storage, direct application of an external power source to the rotary shaft, and an induced electricity generated in the shaft with the use of a rectifier power source. Excessive generation of the shaft voltage causes a shaft current to flow through each bearing portion, resulting in wear in the bearings, damage to the rotary shaft, and blackening of lubricating oil, or in some cases, damage to or scorching of the bearing.
Various approaches have been taken to avoid the above-mentioned problems. For example, Japanese Utility Model Registration Application Laying-open Gazette No. 60-161721 discloses a technique of insulating a bearing with a ceramic layer. In this application, the bearing is treated with a ceramic surface finishing process to form a ceramic layer thereon. Synthetic plastic resin having elasticity and viscosity is then poured into porous portions of the ceramics in order to reinforce the ceramic layer. Japanese Patent Application Laying-open Gazette No. 59-103023 discloses a technique of forming an insulating coating of inorganic compounds with a thermal spraying method. Japanese Patent Application No. 53-80549 discloses a technique of forming an insulating coating with a baking or a coating method. Japanese Utility Model Registration Application Laying-open Gazette No. 59-173467 discloses an insulating technique using a ceramic coating formed by a thermal spraying method.
Various techniques for insulating an axle bearing portion have been known in the art. For example, Japanese Utility Model Registration Application Laying-open Gazette No. 54-105408 discloses a technique of forming an insulating polymer member to cover an outer peripheral surface and a side surface of an outer ring. Japanese Utility Model Registration Application Laying-open Gazette No. 58-33820 discloses a technique of interposing an electrical insulating member between an outer ring and a saddle. In this application, this electrical insulating member is formed in such a manner that a coating member of polymer elastic material is sandwiched between thin metallic plates.
When rotary bearings used in main electric motors for rolling stocks are not electrically insulated, shaft current flows between a main electric motor frame and an armature shaft through a metallic bearing. As a result, the bearing is subject to damage or scorching. The aforementioned prior art techniques attempted to interrupt the shaft current in order to protect the bearing portions in the main electric motors from damage.
Bearings used in main electric motors for rolling stocks are generally installed onto end covers (mounting portions of bearings) of the main electric motors. Accordingly, installation work for installing a bearing on a cover is necessary. In this sense, higher dimensional accuracy is required for the bearings. However, this higher dimensional accuracy cannot be obtained easily when it comes to bearings having insulating members. As a result, the overall dimensional accuracy in installation is lowered when installing these bearings with low dimensional accuracy.
A ceramic-treated bearing has succeeded in improving its dimensional accuracy, but not in overcoming the brittleness of the ceramic itself. In addition, the insulating capacity of the bearing from the cover of the main electric motors can be evaluated only after installation.
In terms of improving the dimensional accuracy, a technique using polymer insulating materials alone is disadvantageous, since it is impossible to raise the dimensional accuracy of the polymer member itself. Furthermore, an insulating material has been made into the form of a thin layer in order to lessen the amount of distortion when a load acts upon the bearing. But this tends to cause damage to the polymer member or ceramic materials upon installation or removal of the bearing.